Add array len parse ui tests

Author: dtolnay

tests/ui/array_len_expr.rs

@@ -0,0 +1,9 @@
+#[cxx::bridge]
+mod ffi {
+    unsafe extern "C++" {
+        fn arraystr() -> [String; "13"];
+        fn arraysub() -> [String; 15 - 1];
+    }
+}
+
+fn main() {}

tests/ui/array_len_expr.stderr

@@ -0,0 +1,11 @@
+error: array length must be an integer literal
+ --> $DIR/array_len_expr.rs:4:35
+  |
+4 |         fn arraystr() -> [String; "13"];
+  |                                   ^^^^
+
+error: unsupported expression, array length must be an integer literal
+ --> $DIR/array_len_expr.rs:5:35
+  |
+5 |         fn arraysub() -> [String; 15 - 1];
+  |                                   ^^^^^^